The invention relates to the hydrogenation of aromatic compounds, in particular the preparation of alicyclic polycarboxylic acids or their esters by core hydrogenation of the corresponding aromatic polycarboxylic acids or their esters, and also to catalysts suitable therefor.
Alicyclic polycarboxylic esters, for example the esters of cyclohexane-1,2-dicarboxylic acid, are used as lubricant components and as assistants in metal processing. They also find use as plasticizers for polyolefins and for PVC.
For plasticizing PVC, esters of phthalic acid are used predominantly, for example the dibutyl, dioctyl, dinonyl or didecyl esters. Since the use of these phthalates has been discussed with increasing controversy in recent times, their use in plastics could be restricted. Alicyclic polycarboxylic esters, some of which have already been described as plasticizers for plastics in the literature, could then be available as suitable substitutes.
In most cases, the most economical route for preparing alicyclic polycarboxylic esters is the core hydrogenation of the corresponding aromatic polycarboxylic esters, for example the abovementioned phthalates. Some, processes already exist for this purpose:
U.S. Pat. No. 5,286,898 and U.S. Pat. No. 5,319,129 describe processes with which dimethyl terephthalate may be hydrogenated over supported Pd catalysts which are doped with Ni, Pt and/or Ru at temperatures greater than or equal to 140° C. and a pressure between 50 and 170 bar to give the corresponding dimethyl hexahydroterephthalate.
U.S. Pat. No. 3,027,398 discloses the hydrogenation of dimethyl terephthalate over supported Ru catalysts at from 110 to 140° C. and from 35 to 105 bar.
DE 28 23 165 discloses the hydrogenation of aromatic carboxylic esters over supported N1, Ru, Rh and/or Pd catalysts to the corresponding alicyclic carboxylic esters at from 70 to 250° C. and from 30 to 200 bar. A macroporous support having an average pore size of 70 nm and a BET surface area of approx. 30 m2/g is used.
WO 99/32427 and WO 00/78704 disclose processes for hydrogenating benzenepolycarboxylic esters to the corresponding alicyclic compounds. Supported catalysts are used which comprise a metal of transition group, VIII alone or together with at least one metal of transition group I or VII of the Periodic Table and have macropores. A preferred metal of transition group VIII used is ruthenium. To hydrogenate, three different catalyst types are used which differ substantially by their average pore diameter and BET surface areas.
Catalyst I: average pore. diameter greater than 50 nm and BET surface area less than 30 m2/g
Catalyst II: average pore diameter from 5 to 20 nm and BET surface area greater than 50 m2/g
Catalyst III: average-pore diameter greater than 100 nm and BET surface area less than 15 m2/g
In addition to the pore diameter, the pore volume formed by pores of a certain diameter is specified. The support materials used in the preparation of catalyst II have a pore distribution in which from approx. 5 to approx. 50% of the pore volume by macropores (diameter from approx. 50 nm to 10 000 nm) and from approx. 70 to approx. 90% of the pore volume by mesopores (diameter from approx. 2 to 50 nm). The average pore diameter is between approx. 5 and 20 nm.
The activity and selectivity of hydrogenation catalysts depends on their surface properties such as pore size, BET surface area or surface concentration of the active metals.
The catalysts used for the core hydrogenation of aromatic carboxylic acids or their esters should allow a high reaction rate, only generate a small proportion of by-products and have a long on-stream time.
In a continuously operated process, a catalyst is exposed to mechanical, thermal and chemical stresses which change the pore size and the BET surface area and thus reduce the activity and selectivity of this catalyst.
In addition to mechanical abrasion, many catalysts also exhibit an expansion of the pore volumes and diameter due to acid digestion.
Aromatic polycarboxylic esters frequently contain small amounts of carboxylic acids, and traces of acid additionally form during the core hydrogenation of esters. Partial esters of polycarboxylic acids or polycarboxylic acids themselves are acidic as a consequence of their structure. Therefore, a hydrogenation catalyst suitable for a continuous process should be resistant to acid even at relatively high temperatures under the hydrogenation conditions.
The surface properties of the catalysts are also responsible for their reactivity. The existing catalysts are in need of improvement in this respect.